Methods to avoid over-developed aluminized coatings during hot stamping line stoppages

ABSTRACT

A method of treating a blank is provided. The method includes moving a blank through a first section of a furnace at an inter-critical temperature and through a second section of the furnace at a critical temperature greater than the inter-critical temperature before hot stamping the blank. Movement of the blank from the first section to the second section of the furnace is delayed during a hot stamping line stoppage. The blank is in the first section of the furnace for a first time period and in the second section of the furnace a second time period less than the first time period. Also, the first section of the furnace may have a first length and in the second section of the furnace may have a second length that is less than the first length.

FIELD

The present disclosure relates to hot forming steels, and particularly,to hot stamping of coated press hardenable steel.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

Hot stamping and forming parts out of a press hardenable steel (PHS)generally requires heating a PHS blank into the intercritically (e.g.,between 750° C. and 850° C.) or austenitic phase region (e.g., above900° C.) of the steel, hot stamping the PHS blank, and cooling the hotstamped PHS blank such that a hot stamped PHS part with a martensiticmicrostructure is provided. However, heating the PHS blank into theaustenitic phase region results in undesirable oxidation of the PHS.Accordingly, PHS material is typically provided by steel suppliers withan oxidation resistant coating, for example, an aluminum-silicon alloycoating formed by dipping or passing the PHS through a liquidaluminum-silicon alloy bath. Such coated PHS is often referred to asaluminized PHS. Upon heating an aluminized PHS blank, diffusion betweenthe aluminum-silicon alloy coating and the PHS results in at least twolayers, one of which is an interdiffusion layer (IDL) between the PHSand an outer aluminized alloy layer. Unfortunately, the IDL affectswelding, particularly resistance welding, of a hot stamped part. Also,studies have shown that an IDL with a thickness equal to or greater than16 micrometers (μm) prevents resistance welding of hot stampedaluminized PHS parts.

The present disclosure addresses the issues of IDL thickness among otherissues related to hot stamping of steels that have an aluminizedcoating.

SUMMARY

This section provides a general summary of the disclosure and is not acomprehensive disclosure of its full scope or all of its features.

In one form of the present disclosure, a method of treating a blank isprovided. The method comprises moving a blank through a first section ofa furnace at an inter-critical temperature and through a second sectionof the furnace at a critical temperature greater than the inter-criticaltemperature; and hot stamping the blank, wherein movement of the blankfrom the first section to the second section of the furnace is delayedduring a hot stamping line stoppage.

In some aspects of the present disclosure, the blank is in the firstsection of the furnace for a first time period and in the second sectionof the furnace a second time period less than the first time period. Insuch aspects, the first time period is at least 1.2 times greater thanthe second time period, for example, at least 1.5 times greater or atleast 2.0 times greater than the second time period. In the alternative,or in addition to, the first section of the furnace has a first lengthand the second section of the furnace has a second length less than thefirst length. For example, in some aspects of the present disclosure,the first length is at least 1.2 times greater than the second length,for example, at least 1.5 times greater or at least 2.0 times greaterthan the second length.

In some aspects of the present disclosure, the blank is a coated presshardenable steel and the inter-critical temperature is between 725° C.and 825° C., and the critical temperature is above 910° C. In suchaspects, the inter-critical temperature may be between 750° C. and 800°C., and the critical temperature may be above 920° C.

In some aspects of the present disclosure, the blank is positioned inthe first section of the furnace at the inter-critical temperature for atime period up to 1200 seconds before moving to the second section ofthe furnace and being hot stamped. Also, the hot stamped blank comprisesan inter-diffusion layer with a thickness less than 16 μm.

In other aspects of the present disclosure, the blank is positioned inthe first section of the furnace at the inter-critical temperature for atime period up to 1800 seconds before moving to the second section ofthe furnace and being hot stamped, and the hot stamped blank comprisesan inter-diffusion layer with a thickness less than 16 μm.

In still other aspects of the present disclosure, the blank ispositioned in the first section of the furnace at the inter-criticaltemperature for a time period up to 2400 seconds before moving to thesecond section of the furnace and being hot stamped, and the hot stampedblank comprises an inter-diffusion layer with a thickness less than 16μm.

In some aspects of the present disclosure, the blank is formed fromaluminized press hardenable steel, it is in the first section of thefurnace during the hot stamping line stoppage for a time period up to1800 seconds. The blank moves from the first section to the secondsection and is hot stamped after the hot stamping line stoppage is overand the hot stamped blank comprises an inter-diffusion layer with athickness less than 16 μm.

In another form of the present disclosure, a method of treating aplurality of blanks during stoppage of a hot stamping line is provided.The method comprises heating a first section of a furnace to aninter-critical temperature between 725° C. and 825° C., heating a secondsection of the furnace to a critical temperature between 910° C. and950° C., moving a plurality of blanks on rollers (e.g. rollers of aroller hearth furnace) through the first section and the second sectionof the furnace to a hot stamping press, and hot stamping the pluralityof blanks. When the rollers stop moving during stoppage of the hotstamping line, a subset of blanks in the first section of the furnace donot move into the second section of the furnace. When the rollers startmoving after the stoppage of the hot stamping line is over, the subsetof blanks in the first section of the furnace move into and through thesecond section of the furnace and to the hot stamping press.

In some aspects of the present disclosure, the plurality of blanks isformed from aluminized press hardenable steel and the subset of blankshot stamped by the hot stamping press after the stoppage of the hotstamping line is over comprise an inter-diffusion layer thickness lessthan 16 μm. Also, the stoppage of the hot stamping line is for a time upto 1800 seconds.

In yet another form of the present disclosure, a method of hot stampinga blank after stoppage of a hot stamping line is provided. The methodcomprises heating a blank formed from a coated press hardenable steel ina first section of a roller hearth furnace to an inter-criticaltemperature for a first time period and heating the blank in a secondsection of the roller hearth furnace to a critical temperature greaterthan the inter-critical temperature for a second time period less thanthe first time period. The blank stops moving from the first section tothe second section of the roller hearth furnace during stoppage of thehot stamping line. Also, the blank moves from the first section to thesecond section of the roller hearth furnace after the stoppage of thehot stamping line is over. The blank is hot stamped after it movesthrough the second section of the furnace. In some aspects of thepresent disclosure, the blank is positioned in the first section of theroller hearth furnace for up to 2400 seconds and the hot stamped blankhas an inter-diffusion layer less than 16 μm. Also, the inter-criticaltemperature is between 725° C. and 825° C., and the critical temperatureis above 910° C.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

DRAWINGS

In order that the disclosure may be well understood, there will now bedescribed various forms thereof, given by way of example, referencebeing made to the accompanying drawings, in which:

FIG. 1 schematically depicts a baseline hot stamping line for producingparts from aluminized press hardenable steel (PHS);

FIG. 2 graphically depicts a baseline heating schedule (profile) for thehot stamping line in FIG. 1;

FIG. 3 schematically depicts a hot stamping line for producing partsfrom aluminized press hardenable steel (PHS) according to the teachingsof the present disclosure;

FIG. 4 graphically depicts a heating profile for the hot stamping linein FIG. 3 according to the teachings of the present disclosure;

FIG. 5 is a flow chart of a method of treating a blank according to theteachings of the present disclosure;

FIG. 6 is a flow chart of a method of treating a plurality of blanksduring stoppage of a hot stamping line according to the teachings of thepresent disclosure;

FIG. 7 is a micrograph of a press hardenable steel (PHS) with analuminized coating following a baseline heat treatment graphicallydepicted in FIG. 2;

FIG. 8A is a micrograph of a PHS with an aluminized coating following 10minutes of heat treatment at an inter-critical temperature T_(IC)graphically depicted in FIG. 4 according to the teachings of the presentdisclosure;

FIG. 8B is a micrograph of a boron steel with an aluminized coatingfollowing 20 minutes of heat treatment at an inter-critical temperatureT_(IC) graphically depicted in FIG. 4 according to the teachings of thepresent disclosure;

FIG. 8C is a micrograph of a boron steel with an aluminized coatingfollowing 30 minutes of heat treatment at an inter-critical temperatureT_(IC) graphically depicted in FIG. 4 according to the teachings of thepresent disclosure;

FIG. 8D is a micrograph of a boron steel with an aluminized coatingfollowing 40 minutes of heat treatment at an inter-critical temperatureT_(IC) graphically depicted in FIG. 4 according to the teachings of thepresent disclosure;

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses. Itshould be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features.Examples are provided to fully convey the scope of the disclosure tothose who are skilled in the art. Numerous specific details are setforth such as types of specific components, devices, and methods, toprovide a thorough understanding of variations of the presentdisclosure. It will be apparent to those skilled in the art thatspecific details need not be employed and that the examples providedherein, may include alternative embodiments and are not intended tolimit the scope of the disclosure. In some examples, well-knownprocesses, well-known device structures, and well-known technologies arenot described in detail.

The present disclosure addresses the issues of interdiffusion layer(IDL) thickness and coating thickness increases due to total furnacetime and other issues related to hot stamping steels that have analuminized coating.

Referring now to FIG. 1, a baseline hot stamping line 10 for producingparts from aluminized press hardenable steel (PHS) is schematicallydepicted. In one form of the present disclosure, the baseline hotstamping line 10 generally includes a furnace transfer station 100, afurnace 110, a hot stamping transfer station 120, a hot stamping station130, and a post-hot stamping transfer station 140. The furnace transferstation 100 transfers aluminized press hardenable steel (PHS) blanks 52from a stack of aluminized PHS blanks 50 to a conveyer line 112 of thefurnace 110. One non-limiting example of the furnace 110 is a rollerhearth furnace and the conveyer line 112 is a plurality of rollers. Theconveyer line 112 moves the aluminized PHS blanks 52 from a first end114 to a second end 116 of the furnace 110 where the hot stampingtransfer station 120 moves the aluminized PHS blanks 52 to the hotstamping station 130. The aluminized PHS blanks 52 are hot stamped atthe hot stamping station 130 to form a hot stamped part 54 and thenremoved therefrom and moved to a subsequent station (not shown) by thepost-hot stamping transfer station 140.

It should be understood that the thickness of an IDL of an aluminizedPHS blank 52 prior to being hot stamped at the hot stamping station 130is a function of its distance-temperature history (also referred toherein as “distance-temperature profile”). One example ofdistance-temperature profile for a plurality of aluminized PHS blanks 52moving along the baseline hot stamping line 10 is graphically depictedin FIG. 2. In such an example, the thickness of the IDL is a function ofthe temperature T_(F) of the furnace 110 (also referred to herein as the“furnace temperature”), residence (dwell) time 22 of the aluminized PHSblank 52 in the furnace temperature 110 (also referred to herein as the“dwell time”), and total time 24 of the aluminized PHS blank 52 in thefurnace 110 also referred to herein as the “total furnace time”). Asused herein, “total furnace time” refers to the time 20 to heat analuminized PHS blank 52 from room temperature T_(R) to furnacetemperature T_(F) plus the dwell time 22. It should also be understoodthat the temperature of the aluminized PHS blank 52 decreases afterremoval from the furnace 110, during hot stamping at the hot stampingstation 130 and during cooling after being hot stamped as graphicallydepicted by section 26 in FIG. 2.

Unfortunately, there are issues which cause stoppage of the baseline hotstamping line 10 and thereby result in aluminized PHS blanks 52exceeding 10 minutes of dwell time 22 at the critical temperature T_(C).Also, these delays turn the affected aluminized PHS blanks 52 intoproduction waste because they are rendered unsuitable for furtherprocessing as the weldability of the blanks is greatly reduced due tothe excessive IDL thickness and porous coating.

As noted above, an IDL thickness equal to or greater than 16 μm reducesthe weldability of the aluminized PHS blank 52 to about zero. As such,current production specifications limit the total furnace time 24 suchthat the IDL thickness is less than 16 μm. For example, depending on thethickness of the PHS blank 52 and the type of oxidation resistantcoating thereon, total furnace time 24 is limited to between 3 to 10minutes. That is, if aluminized PHS blanks 52 are held in the furnacefor a time longer than the prescribed dwell time 22, e.g., due astoppage of the hot stamping line 10, such aluminized PHS blanks 52 havean IDL thickness greater than 16 μm and are typically scrapped. Thistime at temperature dependent scrapping of blanks increases the expenseof the hot stamping process due to lower material yields, wasted energy,wasted labor, etc.

Referring now to FIGS. 3 and 4, a hot stamping line 10′ and adistance-temperature profile according to the teachings of the presentdisclosure are shown. Particularly, the furnace transfer station 100transfers aluminized press hardenable steel (PHS) blanks 52 from a stackof aluminized PHS blanks 50 to the conveyer line 112 of the furnace 110.The conveyer line 112 moves the aluminized PHS blanks 52 from the firstend 114 and through a first section 111 of the furnace 110 at aninter-critical temperature T_(IC), through a second section 113 of thefurnace 110 at a critical temperature T_(C), and to the second end 116of the furnace 110. In some aspects of the present disclosure the firstsection of the furnace 110 extends from the first end 114 to a divider115 and the second section of the furnace 113 extends from the divider115 to the second end 116. In such aspects, the divider 115 may be abaffle, an insulated panel, and the like. In the alternative, thedivider 115 may simply represent a change in the power settings ofheater element or burners (not shown) positioned in the first section111 versus the heater elements or burners positioned in the secondsection 113.

During movement of the aluminized PHS blank 52 through the first section111 of the furnace 110, the aluminized PHS steel blank is heated to theinter-critical temperature T_(IC) during a first transient time period30 and held at the inter-critical temperature T_(IC) for a first timeperiod 32. Upon reaching the second section 113 of the furnace 110, thealuminized PHS blank 52 is heated to the critical temperature TC duringa second transient time period 34 and held at the critical temperatureTC for a second time period 36. When the aluminized PHS blank 52 reachesthe second end 116 of the furnace 110 the hot stamping transfer station120 moves the aluminized PHS blanks 52 to the hot stamping station 130where it is hot stamped and quenched to form a hot stamped PHS part 54.Thereafter, post-hot stamping transfer station 140 transfers the hotstamped PHS part 54 to a subsequent station (not shown).

The inter-critical temperature T_(IC) corresponds to a ferrite plusaustenite microstructure in the aluminized PHS blank 52 and the T_(C)corresponds to a fully austenite microstructure in the aluminized PHSblank 52. The T_(C) and T_(IC) are dependent upon the composition of theboron steel. The inter-critical temperature T_(IC) is sufficiently highsuch that iron diffuses from the PHS steel into the aluminum-siliconalloy coating to form the eutectic AlSi₁₀Fe₃ alloy layer in a short timeframe (e.g., seconds). The solidification temperature of the eutecticAlSi₁₀Fe₃ alloy layer is greater than the melting temperature of thealuminum-silicon alloy layer thereby providing a solid (i.e., notliquid) aluminized coating on the aluminized PHS blank 52 as it movesthrough the furnace 110. Also, the solid eutectic AlSi₁₀Fe₃ alloy layeris not removed from the aluminized PHS blank 52 and transferred to theconveyer line 112 (e.g., rollers). Heating the aluminized PHS blank 52to the critical temperature T_(C), i.e., heating the aluminized PHSblank 52 into the fully austenitic phase region, followed by cooling andquenching in the region 26 graphically depicted in FIG. 4, transformsthe austenite into martensite, martensite plus retained austenite and/ormartensite and bainite. Accordingly, the distance-temperature profilegraphically depicted in FIG. 4 allows for hot stamping of aluminized PHSblanks 52 such that high strength parts are formed. However, and unlikethe distance-temperature profile in FIG. 2, the distance-temperatureprofile graphically depicted in FIG. 4 allows for the aluminized PHSblanks 52 to remain in the furnace 110 for times greater than 10 minuteswithout developing an IDL greater than 16 μm. That is, in the event of astoppage of the hot stamping line 10, diffusion between the PHS steelblank and the eutectic AlSi₁₀Fe₃ alloy layer at the inter-criticaltemperature T_(IC) is such that more than 10 minutes, e.g., more than 30minutes, is needed before the IDL is equal to or greater than 16 μm asdescribed in greater detail below.

Referring now referring to FIG. 5, a method 60 of treating a coated PHSblank is provided. At step 62, the method 60 comprises moving a coatedPHS blank through a first section of a furnace at an inter-criticaltemperature and through a second section of the furnace at a criticaltemperature greater than the inter-critical temperature. The coated PHSblank is hot stamped at step 64 and movement of the blank from the firstsection to the second section of the furnace is delayed during a hotstamping line stoppage.

Referring now to FIG. 6, a method 70 of treating a plurality ofaluminized PHS blanks during stoppage of a hot stamping line isprovided. The method 70 includes heating a first section of a furnace toan inter-critical temperature between 725° C. and 825° C. at step 71 andheating a second section of the furnace to a critical temperaturebetween 910° C. and 950° C. at step 72. A plurality of aluminized PHSblanks are moved through the first section and the second section of thefurnace on a conveyor belt to a hot stamping press at step 73. At step74, the conveyor belt is stopped for a time period up to 40 minutesduring a hot stamping line stoppage such that aluminized PHS blanks inthe first section of the furnace do not move into the second section ofthe furnace. The conveyor belt is re-started after the hot stamping linestoppage is over (i.e., the hot stamping line is moving again) at step75 and the aluminized PHS blanks held in the first section are moved tothe second section of the furnace and to a hot stamping station. At step76, the aluminized PHS blanks held in the first section of the furnacefor the time period up to 40 minutes are hot stamped such that hotstamped aluminized PHS parts are provided. It should be understood thatthe hot stamped aluminized PHS parts formed from the aluminized PHSblanks held in the first section for up to 40 minutes have an IDLthickness of less than 16 μm and thereby can be successfully resistancewelded. It should also be understood that the aluminized PHS blanks heldin the first section for up to 40 minutes exhibited desired mechanicalproperties that meet or exceed predefined strength, ductility and/orimpact resistance levels.

EXAMPLES

Samples of aluminized PHS were subjected to distance-temperatureprofiles as graphically depicted in FIG. 4 for first time periods 32ranging from 10 to 40 minutes and a second time period 36 equal to 3minutes. The aluminized PHS material was obtained from the companyARCELORMITTAL™ with the coating and PHS compositional ranges shown inTable 1 below. It should be understood that other alloys, for exampleother PHSs currently available, PHSs currently being developed but notyet commercially available, and PHSs yet to be developed, can be usedwith the methods disclosed herein and thereby fall within the scope ofthe teachings of the present disclosure.

TABLE 1 Element Min. wt. % Max. wt. % Aluminized coating Iron (Fe) 0 ≤3Silicon (Si) >0 ≤10 Aluminum (Al) Balance Boron Steel Aluminum (Al) 0.020.06 Boron (B) 0 0.005 Carbon (C) 0.2 0.25 Chromium (Cr) 0 0.35 Copper(Cu) 0 0.2 Manganese (Mn) 1.1 1.4 Molybdenum (Mo) 0 0.35 Nitrogen (N) 00.009 Phosphorus (P) 0 0.025 Silicon (Si) 0 0.5 Sulfur (S) 0 0.008Titanium (Ti) 0.02 0.05 Iron (Fe) Balance plus impurities

The inter-critical temperature was between 750° C. and 800° C. and thecritical temperature was 930° C. The samples were metallographicallyprepared and examined using optical microscopy and compared to abaseline PHS sample subjected to the distance-temperature profilegraphically depicted in FIG. 2 with a dwell time 22 of less than 10minutes and a critical temperature T_(C) of 930° C.

Referring now to FIG. 7, an optical microscopy image (also referred toherein as a “micrograph”) of the baseline aluminized coating on the PHSsample subjected to the distance-temperature profile graphicallydepicted in FIG. 2 with a dwell time 22 of less than 5 minutes is shown.As shown in FIG. 7, the average IDL thickness was about 5 μm and theouter eutectic AlSi₁₀Fe₃ alloy layer was about 26.6 μm. Given that theIDL thickness is less than 16 μm, such an aluminized PHS sample issuitable for hot stamping and subsequent resistance welding.

Referring now to FIGS. 8A-8D, micrographs of the PHS samples subjectedto the distance-temperature profiles graphically depicted in FIG. 4 areshown. Particularly, FIG. 8A-8D are micrographs of aluminized PHSsamples subjected to the inter-critical temperature T_(IC) for a firsttime period 32 equal to 10 minutes, 20 minutes, 30 minutes, and 40minutes, respectively, and the critical temperature T_(C) for a secondtime period equal to 3 minutes. The average total coating thickness,average IDL thickness, and average eutectic AlSi₁₀Fe₃ alloy layerthickness (labeled as “Ave. Coating Thickness”) for all of thealuminized PHS samples are provided below in Table 2. As shown in Table2, all of the aluminized PHS samples subjected to distance-temperatureprofiles graphically depicted in FIG. 4 had an IDL thickness less than16 μm, even the aluminized PHS sample subjected to the inter-criticaltemperature T_(IC) for 40 minutes. Accordingly, in the event of astoppage of the hot stamping line 10′ (FIG. 3) greater than 10 minutes,aluminized PHS blanks 52 held in the first section 111 of the furnace110 can be used to produce hot stamped PHS parts 54 and thereby notscrapped.

TABLE 2 Time in Average Total Ave. IDL Ave. Coating furnace CoatingThickness Thickness (minutes) Thickness (μm) (μm) (μm) FIG. 7 5 31.6 526.6 FIG. 8A 10 38.5 6.7 31.8 FIG. 8B 20 36.5 8.1 28.4 FIG. 8C 30 37.77.9 29.8 FIG. 8D 40 35.7 9.4 26.3

In some aspects of the present disclosure, a longer roller hearthfurnace compared to traditional roller hearth furnaces used in hotstamping lines is used in the methods disclosed herein. It should beunderstood that the increase in length of the roller hearth furnace mayincrease the cost of the roller hearth furnace. However, the additionallength of travel at temperature in the longer roller hearth furnaceprovides an increase in the number of blanks per unit area, feed rate ofthe blanks through the furnace, the number of temperature zones, and/orthe temperature per zone. The inventors have modelled the process innon-roller hearth furnaces which are also within the scope of thepresent disclosure. The present disclosure is also applicable tozinc-coated press hardened steels.

The present disclosure reduces the scrap rate, blanks will stillexperience quality control following an unplanned line stoppage, howeverthe scrap rate has a potential of being 80% with some models placing thescrap rate below 50%, 30%, or 10%. However, even an 80% scrap rate is amarked improvement over the current 100% scrap rate.

Although the terms first, second, third, etc. may be used to describevarious elements, components, regions, layers and/or sections, theseelements, components, regions, layers and/or sections, should not belimited by these terms. These terms may be only used to distinguish oneelement, component, region, layer and/or section, from another element,component, region, layer and/or section. Terms such as “first,”“second,” and other numerical terms when used herein do not imply asequence or order unless clearly indicated by the context. Thus, a firstelement, component, region, layer or section, could be termed a secondelement, component, region, layer or section without departing from theteachings of the example forms. Furthermore, an element, component,region, layer or section may be termed a “second” element, component,region, layer or section, without the need for an element, component,region, layer or section termed a “first” element, component, region,layer or section.

As used herein, the phrase at least one of A, B, and C should beconstrued to mean a logical (A OR B OR C), using a non-exclusive logicalOR, and should not be construed to mean “at least one of A, at least oneof B, and at least one of C.

Unless otherwise expressly indicated, all numerical values indicatingmechanical/thermal properties, compositional percentages, dimensionsand/or tolerances, or other characteristics are to be understood asmodified by the word “about” or “approximately” in describing the scopeof the present disclosure. This modification is desired for variousreasons including industrial practice, manufacturing technology, andtesting capability.

The terminology used herein is for the purpose of describing particularexample forms only and is not intended to be limiting. The singularforms “a,” “an,” and “the” may be intended to include the plural formsas well, unless the context clearly indicates otherwise. The terms“including,” and “having,” are inclusive and therefore specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. The method steps, processes, andoperations described herein are not to be construed as necessarilyrequiring their performance in the particular order discussed orillustrated, unless specifically identified as an order of performance.It is also to be understood that additional or alternative steps may beemployed.

The description of the disclosure is merely exemplary in nature and,thus, examples that do not depart from the substance of the disclosureare intended to be within the scope of the disclosure. Such examples arenot to be regarded as a departure from the spirit and scope of thedisclosure. The broad teachings of the disclosure can be implemented ina variety of forms. Therefore, while this disclosure includes particularexamples, the true scope of the disclosure should not be so limitedsince other modifications will become apparent upon a study of thedrawings, the specification, and the following claims.

What is claimed is:
 1. A method of treating a blank comprising: moving ablank through a first section of a furnace at an inter-criticaltemperature and through a second section of the furnace at a criticaltemperature greater than the inter-critical temperature; and hotstamping the blank, wherein movement of the blank from the first sectionto the second section of the furnace is delayed during a hot stampingline stoppage.
 2. The method according to claim 1, wherein the blank isin the first section of the furnace for a first time period and in thesecond section of the furnace a second time period less than the firsttime period.
 3. The method according to claim 2, wherein the first timeperiod is at least 1.2 times greater than the second time period.
 4. Themethod according to claim 1, wherein the first section of the furnacehas a first length and the second section of the furnace has a secondlength less than the first length.
 5. The method according to claim 1,wherein the first length is at least 1.2 times greater than and thesecond length.
 6. The method according to claim 1, wherein the blank isformed from coated press hardenable steel.
 7. The method according toclaim 1, wherein the blank is formed from aluminized press hardenablesteel, the inter-critical temperature is between 725° C. and 825° C.,and the critical temperature is above 910° C.
 8. The method according toclaim 1, wherein the blank is formed from aluminized press hardenablesteel, the inter-critical temperature is between 750° C. and 800° C.,and the critical temperature is above 920° C.
 9. The method according toclaim 8, wherein the blank is positioned in the first section of thefurnace at the inter-critical temperature for a time period up to 1200seconds before moving to the second section of the furnace and being hotstamped.
 10. The method according to claim 9, wherein the hot stampedblank comprises an inter-diffusion layer with a thickness less than 16μm.
 11. The method according to claim 8, wherein the blank is positionedin the first section of the furnace at the inter-critical temperaturefor a time period up to 1800 seconds before moving to the second sectionof the furnace and being hot stamped, and the hot stamped blankcomprises an inter-diffusion layer with a thickness less than 16 μm. 12.The method according to claim 8, wherein the blank is positioned in thefirst section of the furnace at the inter-critical temperature for atime period up to 2400 seconds before moving to the second section ofthe furnace and being hot stamped, and the hot stamped blank comprisesan inter-diffusion layer with a thickness less than 16 μm.
 13. Themethod according to claim 1, wherein: the blank is formed fromaluminized press hardenable steel; the blank is in the first section ofthe furnace during the hot stamping line stoppage for a time period upto 1800 seconds; the blank moves from the first section to the secondsection and is hot stamped after the hot stamping line stoppage is over;and the hot stamped blank comprises an inter-diffusion layer with athickness less than 16 μm.
 14. The method according to claim 13, whereinthe inter-critical temperature is between 725° C. and 825° C., and thecritical temperature is between 910° C. and 950° C.
 15. The methodaccording to claim 1, wherein: the blank is formed from aluminized presshardenable steel; the inter-critical temperature is between 750° C. and800° C., and the critical temperature is between 920° C. and 940° C. theblank is in the first section of the furnace during the hot stampingline stoppage for a time period up to 2400 seconds; the blank moves fromthe first section to the second section and is hot stamped after the hotstamping line stoppage is over; and the hot stamped blank comprises aninter-diffusion layer with a thickness less than 16 μm.
 16. A method oftreating a plurality of blanks during stoppage of a hot stamping line,the method comprising: heating a first section of a furnace to aninter-critical temperature between 725° C. and 825° C.; heating a secondsection of the furnace to a critical temperature between 910° C. and950° C.; and moving a plurality of blanks on a conveyer line through thefirst section and the second section of the furnace to a hot stampingpress and hot stamping the plurality of blanks; wherein: the conveyerline stops moving during stoppage of the hot stamping line such that asubset of blanks in the first section of the furnace do not move intothe second section of the furnace; and the conveyer line starts movingafter the stoppage of the hot stamping line is over such that the subsetof blanks in the first section of the furnace move into and through thesecond section of the furnace and to the hot stamping press.
 17. Themethod according to claim 16, wherein the plurality of blanks is formedfrom aluminized press hardenable steel and the subset of blanks hotstamped by the hot stamping press after the stoppage of the hot stampingline is over comprise an inter-diffusion layer thickness less than 16μm.
 18. The method according to claim 17, wherein the stoppage of thehot stamping line is for a time up to 1800 seconds.
 19. A method of hotstamping a blank after stoppage of a hot stamping line comprising:heating a blank formed from a coated press hardenable steel in a firstsection of a roller hearth furnace to an inter-critical temperature fora first time period; heating the blank in a second section of the rollerhearth furnace to a critical temperature greater than the inter-criticaltemperature for a second time period less than the first time period;stopping the blank from moving from the first section to the secondsection of the roller hearth furnace during stoppage of the hot stampingline; moving the blank from the first section to the second section ofthe roller hearth furnace after the stoppage of the hot stamping line isover; and hot stamping the blank after it moves through the secondsection of the furnace, wherein the blank is positioned in the firstsection of the roller hearth furnace for up to 2400 seconds and the hotstamped blank has an inter-diffusion layer less than 16 μm.
 20. Themethod according to claim 19, wherein the inter-critical temperature isbetween 725° C. and 825° C., and the critical temperature is above 910°C.